feat: make global default timeout 0 (infinite) instead of required 30s

DefaultTimeoutSeconds now means "no timeout" when 0/empty, and that is
the new default, rather than an invalid config forcing a positive
value. runner.RunJob avoids context.WithTimeout with a zero duration
(which would expire immediately) and instead runs on a plain
cancelable context when no timeout is configured. Per-job
TimeoutSeconds inherit semantics are unchanged.

src/app/format.go

@@ -105,11 +105,14 @@ func DisplayOverlapPolicy(job domain.Job, globalPolicy domain.OverlapPolicy) str
 // DisplayTimeout formats a job's effective run timeout for the details panel.
 // When the job sets its own TimeoutSeconds it is shown as-is; when 0 (inherit),
 // the global default is shown with "(global default)" appended, mirroring
-// DisplayOverlapPolicy.
+// DisplayOverlapPolicy. A non-positive global default means no timeout at all.
 func DisplayTimeout(job domain.Job, globalDefault int) string {
 	if job.TimeoutSeconds > 0 {
 		return fmt.Sprintf("%d s", job.TimeoutSeconds)
 	}
+	if globalDefault <= 0 {
+		return "no timeout (global default)"
+	}
 	return fmt.Sprintf("%d s (global default)", globalDefault)
 }